Preparation of thin sections of material from melts of cellulose esters



United States PREPARATION OF THIN SECTIONS OF MATERIAL FROM MELTS OF CELLULOSE ESTERS Reinhold Gaebel, Berlin-Zehlendorf, and Helmut Neugebauer, Berlin-Wannsee, Germany, assignors, by mesne assignments, to Hercules Powder Company, Wilmington, Del., a corporation of Delaware No Drawing. Application March 30, 1954 Serial No. 419,922

Claims priority, application Germany July 15, 1953 3 Claims. (Cl. 18-54) strength, i. e., when the fiber is tied into a knot and weighed to the breaking point, the measured breaking weight is only a small portion of the breaking weight in the untied state, viz., 1-10%, while a suitable thread or fiber is expected to have at least 60% of the breaking weight in the untied state.

An attempt has been made to compensate for this deficiency in knot-strength by previously admixing plasticizers or solvents with the cellulose acetate to be spun, thereby causing the fibers to become softer. Of course, this involves a reduction in the normal strength, since it is only with relatively high amounts of additives that the knot-strength is rendered adequate. Small quantities of plasticizers are capable of affecting other properties of threads or fibers, but do not alter the knot-strength, and, consequently, do not enter into consideration. Plasticizers, moreover, have the disadvantage that they either wash out in time, or are even injurious.

It has now been discovered that threads or fibers having good knot-strength are obtained by exercising care at the moment of solidification of the fiber structure that the fused cellulose acetate issuing from the jet undergoes no substantial hot-drawing. This calls for a dilierentiation between hot-drawing and cold-drawing. Hotdrawing is the increase in velocity in the axial direction of a mass which is still viscous, and has, therefore, not yet entirely solidified, while cold-drawing is such an increase in velocity of the solidified fiber structure. It is also possible for the rate of the fused mass issuing from the jet to be less than that in the jet hole. In such a case one speaks of a compression.

It is of importance in connection with the present invention to avoid hot-drawing of the still-viscid mass, which may be only very slight. This requirement is, for example, met when the rate of the solidified fiber after the jet is equal to or less than the rate in the hole of the jet. The rate in the hole of the jet is calculated from the hole diameter and the mass extruded in unit time. In the present case the thread of fiber has approximately the thickness of the diameter of the jet hole, or, in the case of compression, it may be thicker.

Conformity between hole diameter and fiber thickness may be obtained, as has already been suggested, by immediately cooling the fiber at the jet by blowing or by extrusion into a cooling liquid; this, however, does not constitute the essential feature of the invention. That atent Kit? is, in such a case, one would be limited by the diameter of the jet hole. Since industrially satisfactory holes of less than 0.04 mm. cannot be prepared, and since such small holes involve ditficulty in connection with the danger of clogging, it would be impossible to prepare textile fibers having a thickness of ca. 0.01 mm. or less.

It has now further been discovered, in connection with the invention, that it is possible to produce fibers having a diameter less than that of the jet hole and having, in spite of this, knot-strength. As was mentioned above, hot-drawing of the viscid mass produces a bad effect on the knot-strength. If care is taken to keep the extruded melt tenuous by means of heat, the extruded liquid fiber becomes thinner in correspondence with the viscosity of the melt, and it contracts, the rate of the liquid increasing rapidly at first, and then gradually more slowly, to a value determined by the viscosity and surface tension of the liquid. According to the invention, this rate cannot be further increased, e. g., by more rapid drawing off by the winding apparatus. This is achieved by elongating the heating jacket (to provide a heat zone) in the immediate vicinity of the jet in order to keep the mass liquid. If the liquid fibers contract on extrusion by, for example, one-fourth, it becomes possible to wind at 16 times the hole velocity without hot-drawing. Naturally, the contraction of the liquid fiber increases with increasing lengths of the heating zone after the jet, with the result that a finer fiber may be spun.

Example 1 Melted cellulose acetate is extruded from a jet having a hole diameter of 0.1 mm. at a rate of ca. 2 cm./sec.

The resulting fiber is drawn Off at a rate of 50 cm. and

wound. The fiber has a knot-strength of 6%.

Example 2 The same fiber is drawn off at a rate of 3 cm./sec.. and wound. The fiber has a knot-strength of and scarcely shows any illumination under the polarizing microscope, while the fiber of Example 1 lights up very brightly, indicating hot-drawing.

Example 3 Examples 1 and 2 are varied in that the mass is made more viscous by means of a lower melting temperature. The fiber now has a rate of ca. 1.5 cm./ sec. after the jet and is wound at this velocity. The measured knot-strength is 60% and the fiber diameter is 0.19 mm, thus thicker than would correspond to the jet hole diameter.

Example 4 A jet having a diameter of 0.2 mm. is not screwed on to the outside of the melt tube as usual, but is screwed onto the melt tube in such a way that the heating jacket extends about 3 cm. beyond the bottom of the jet. The fiber appears outside the heating jacket at a rate of 12 cm./sec. and is wound at this velocity. The fiber has a diameter of only 0.08 mm. and a knot-strength of 60%.

The thread or fibers spun in accordance with this process may be further subjected to a stretching or saponification process. Small quantities of plasticizers, which are not capable of causing any increase in knot-strength per se, may be added to the melt, without altering the principle of the invention. Furthermore, the process is not limited to cellulose acetate, but may be applied to other cellulose esters, in so far as these have a tendency, based on their structure, toward poor knot-strength.

What is claimed and desired to protect by Letters Patent is:

1. A method for the production of filaments of good knot-strength by extrusion of melted cellulose ester through a spinneret, which comprises the following steps: extruding the melted cellulose ester in the form of a molten filament from the spinneret into a heating zone at the bottom of the spinneret, additionally heating the molten filament in the heating zone as it passes therethrough to reduce its surface tension and to thereby reduce its cross-sectional area with corresponding elongation thereof, passing the molten filament so produced from the heating zone and solidifying it by cooling, and collecting the solidified filament at a rate to prevent any substantial drawing of the filament during any of the preceding recited steps.

2. A method as claimed in claim 1 wherein the heating zone is elongated in the direction of travel of the filament.

3. A method as claimed in claim 2 wherein the cellulose ester material is a cellulose acetate.

References Cited in the file of this patent UNITED STATES PATENTS Dreyfus et al. Dec. 1, 1942 Dreyfus Dec. 1, 1942 Hull July 13, 1943 Bent Dec. 7, 1943 FOREIGN PATENTS Great Britain Dec, 9, 1941 

1. A METHOD FOR THE PRODUCTION OF FILAMENTS OF GOOD KNOT-STRENGTH BY EXTRUSION OF MELTED CELLULOSE ESTER THROUGH A SPINNERET, WHICH COMPRISES THE FOLLOWING STEPS: EXTRUDING THE MELTED CELLULOSE ESTER IN THE FORM OF A MOLTEN FILAMENT FROM THE SPINNERET INTO A HEATING ZONE AT THE BOTTOM OF THE SPINNERET, ADDITIONALLY HEATING THE MOLTEN FILAMENT IN THE HEATING ZONE AS IT PASSES THERETHROUGH TO REDUCE ITS SURFACE TENSIOLN AND TO THEREBY REDUCE ITS CROSS-SECTIONAL AREA WITH CORRESPONDING ELONGATION THEREOF, PASSING THE MOLTEN FILAMENT SO PRODUCED FROM THE HEATING ZONE AND SOLIDIFYING IT BY COOLING, AND COLLECTING THE SOLIDIFIED FILAMENT AT A RATE TO PREVENT ANY SUBSTANTIAL DRAWING OF THE FILAMENT DURING ANY OF THE PRECEEDING RECITED STEPS. 